Unity is a shell interface for the GNOME desktop environment developed by Canonical for its Ubuntu operating system. Unity debuted in the netbook edition of Ubuntu 10.10. It is designed to make more efficient use of space given the limited screen size of netbooks, including, for example, a vertical application switcher called the launcher. Unlike GNOME, KDE Software Compilation, or Xfce, Unity is not a collection of applications but designed to be used with existing GTK+ programs.

Ubuntu has traditionally used the full GNOME desktop environment; founder Mark Shuttleworth cited philosophical differences with the GNOME team over the user experience to explain why Ubuntu would use Unity as the default desktop instead of GNOME Shell, beginning April 2011, with Ubuntu Natty Narwhal (11.04).

But it is possible to install Gnome 3 on Ubuntu 11.04 (Natty Narwhal)

GNOME 3 is crammed full of new features. Here are some of the other things that you can look forward to from GNOME 3:
* Side-by-side window tiling to make using several windows simple and easy
* A redesigned file manager
* Redesigned workspaces so user can easily organise your windows
* Major changes under the hood to give user a faster, smoother experience
* A satisfying experience, whatever kind of computer you use: GNOME 3 will feel right at home on netbooks as well as larger machines
* And many more ...

Gnome 3 Installation:
Open up the terminal and type following commands: